Microwave-photonic optical links are commonly used in various communication applications. Typically, a microwave-photonic optical link transmits radio frequency (RF) signals over optical fiber links. RF over optical fiber transmissions (may also be referred to as “RoF” transmissions) are used in many applications, such as broad-band wireless access networks, wireless sensor networks, and radar and satellite communication systems.
One challenge in using optical fiber for RF transmission is that signal degradation may occur from a wide double sideband signal due to dispersion in the optical fiber.
Another challenge is that a large carrier signal level may be greater than a receive detector saturation level and a large carrier may also cause “Stimulated Brillouin Scattering” in long fibers. Stimulated Brillouin Scattering is caused by acoustical phonons generated from continuous-wave electromagnetic waves traveling in an optical fiber. The electromagnetic optical field is coupled to acoustic waves through the process of electrostriction, which sets up an index grating that scatters the incoming optical signal in the backwards direction through Bragg diffraction. This phenomenon is known as Stimulated Brillouin Scattering.
Typically, RF photonic devices use an external modulator for modulating a signal. One commonly used modulator is a lithium-niobate based Mach-Zehnder modulator. The Mach-Zehnder modulator uses a control circuit to keep it at a particular operating point, which has limitations.
It is desirable to have the flexibility to use a Mach-Zehnder modulator or a simple phase modulator, for communication systems using microwave photonic optical links.